Motion imaging systems represent moving images as a sequence of stationary images. These stationary images represent temporal samples of a scene. Different image acquisition technologies use different temporal sampling rates. In a video editing process it is common to combine images acquired at differing temporal sampling rates in the same video presentation. For example a television program may include television images sampled at 59.94 Hz and film material sampled at 24 Hz. The material from the film frames may be presented by repeating film frames in a ‘3:2 sequence’ that comprises a phase-perturbed temporal sampling pattern. These two sampling patterns may occur simultaneously in the same video field or frame—for example in a ‘split-screen’ effect, or where television-originated captions are ‘keyed-into’ film-originated images.
Many image-processing techniques combine information from adjacent images in a sequence of images that represents a moving image; such processes include standards-conversion, de-interlacing, noise reduction and aspect-ratio conversion. These processes can be improved by taking into account the temporal sampling patterns of the image pixels in deciding whether to combine information from adjacent temporal samples.
The problem of detecting the temporal sampling pattern is particularly relevant to the processing of interlaced television images that incorporate film material having a frame-rate lower than the television field-rate. The term ‘film-cadence’, which will be shortened to ‘cadence’ elsewhere in this specification, is commonly used to describe the temporal relationship between spatially equivalent pixels in preceding and succeeding images in a motion-image sequence. Typically the cadence of a pixel can be assigned to one of three categories:                ‘Film 1’ where the corresponding, succeeding-image pixel corresponds to the same temporal sample, and the corresponding, preceding-image pixel corresponds to a preceding temporal sample;        ‘Film 2’ where the corresponding, preceding-image pixel corresponds to the same temporal sample, and the corresponding, succeeding-image pixel corresponds to a succeeding temporal sample; and,        ‘Video’ where the corresponding, preceding and succeeding pixels correspond respectively to preceding and succeeding temporal samples.        
If a pixel corresponds to part of the image sequence that contains no moving objects the cadence is usually irrelevant and cannot be determined by analysis of pixel values. A special additional category of ‘Still’ pixels may be used to cater for this situation.
European patent EP 1 127 455 describes methods for determining cadence by comparing the absolute magnitudes of inter-field difference signals. These difference signals may be ‘corrected’ for the presence of detail by subtracting a measure of detail from them prior to the comparison. The present invention provides improved methods of cadence detection which are particularly suitable for detecting relatively small regions within the image having cadence differing from the remainder of the image.